NROM fabrication method

ABSTRACT

A method of fabricating an oxide-nitride-oxide (ONO) layer in a memory cell to retain charge well in the nitride layer includes the steps of forming a bottom oxide layer on a substrate, depositing a nitride layer and oxidizing a top oxide layer, thereby causing oxygen to be introduced into the nitride layer. Another method includes the steps of forming a bottom oxide layer on a substrate, depositing a nitride layer and oxidizing a portion of a top oxide layer, thereby causing oxygen to be introduced into the nitride layer and depositing a remaining portion of the top oxide layer, thereby assisting in controlling the amount of oxygen introduced into the nitride layer. A further method includes the steps of forming a bottom oxide layer on a substrate, depositing a nitride layer, depositing a portion of a top oxide layer and oxidizing a remaining portion of the top oxide layer, thereby causing oxygen to be introduced into the nitride layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 11/979,183, filed Oct. 31, 2007 which is acontinuation application of U.S. patent application Ser. No. 10/961,398,filed Oct. 12, 2004 which is a continuation application of U.S. patentapplication Ser. No. 09/966,754, filed Oct. 1, 2001 now U.S. Pat. No.6,803,279 which is a continuation application of U.S. patent applicationSer. No. 09/365,369, filed Jul. 30, 1999 now U.S. Pat. No. 6,297,096,which is a continuation-in-part application of U.S. patent applicationSer. No. 08/873,384, filed Jun. 11, 1997 now U.S. Pat. No. 5,966,603,all of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to methods of fabrication ofnitride read only memory (NROM) cells and arrays.

BACKGROUND OF THE INVENTION

FIG. 1, to which reference is made, illustrates a typical prior art NROMcell. This cell includes a substrate 10 in which are implanted a source12 and a drain 14 and on top of which lies an oxide-nitride-oxide (ONO)structure 16 having a layer of nitride 17 sandwiched between two oxidelayers 18 and 20. On top of the ONO structure 16 lies a gate conductor22. Between source 12 and drain 14 is a channel 15 formed under ONOstructure 16.

Nitride section 17 provides the charge retention mechanism forprogramming the memory cell. Specifically, when programming voltages areprovided to source 12, drain 14 and gate conductor 22, electrons flowtowards drain 14. According to the hot electron injection phenomenon,some hot electrons penetrate through the lower section of silicon oxide18, especially if section 18 is thin, and are then collected in nitridesection 17. As is known in the art, nitride section 17 retains thereceived charge, labeled 24, in a concentrated area adjacent drain 14.Concentrated charge 24 significantly raises the threshold of the portionof the channel of the memory cell under charge 24 to be higher than thethreshold of the remaining portion of the channel 15.

When concentrated charge 24 is present (i.e. the cell is programmed),the raised threshold of the cell does not permit the cell to be placedinto a conductive state during reading of the cell. If concentratedcharge 24 is not present, the read voltage on gate conductor 22 canovercome the much lower threshold and accordingly, channel 15 becomesinverted and hence, conductive.

U.S. application Ser. No. 08/861,430 filed Jul. 23, 1996 and owned bythe common inventor of the present invention describes an improved NROMcell, which is programmed in one direction and read in the reversedirection.

It is noted that the threshold voltage Vth of NROM cells is generallyvery sensitive to the voltages Vdrain and Vgate provided on the drain 14and on the gate 22, respectively. Furthermore, U.S. application Ser. No.08/861,430 selects the voltages Vdrain and Vgate are selected in orderto ensure that the charge trapped in a portion of the nitride layer 17remains localized in that portion.

Read only memory cells, including a nitride layer in the gate dielectric(NROM) are described, inter alia, in U.S. Pat. No. 5,168,334 to Mitchellet al. and U.S. Pat. No. 4,173,766 to Hayes.

Mitchell et al. describe two processes to produce the NROM cells. In thefirst process, bit lines are first created in the substrate, after whichthe surface is oxidized. Following the oxidation, the ONO layers areadded over the entire array. Polysilicon word lines are then depositedin rows over the ONO layers. Unfortunately, when an oxide layer is grown(typically under high temperature), the already present bit lines willdiffuse to the side, an undesirable occurrence which limits the extentto which the cell size can be shrunk.

In the second process, the ONO layers are formed over the entire arrayfirst, on top of which conductive blocks of polysilicon are formed. Thebit lines are implanted between the blocks of polysilicon after whichthe ONO layers are etched away from on top of the bit lines. Planarizedoxide is then deposited between the polysilicon blocks after whichpolysilicon word lines are deposited. Mitchell et al. utilize aplanarized oxide since such can be deposited rather than grown. Mitchellet al. cannot grow an oxide over the bit lines since such an oxidationoperation would also grow oxide over the polysilicon blocks and thelatter must be left with a very clean surface in order to connect withthe polysilicon word lines. Unfortunately, planarized oxide is not aclean oxide nor does it seal around the edges of the ONO sections.Furthermore, the planarized oxide adds complexity and cost to theprocess.

Hayes et al. describe an NROM cell having only an oxide-nitride (ON)layer. The cells in the array are created by forming layers of oxide,nitride and polysilicon (the latter to produce the gate) one afteranother and then patterning and etching these layers to form the oncells. The uncapped nitride in each cell does not hold charge well inboth the vertical and lateral directions. Due to hole and hot electronconduction within the nitride, the charge to be stored will flowvertically towards the gate covering it unless the nitride is thick andwill flow laterally in the nitride in response to lateral electricfields.

SUMMARY OF THE PRESENT INVENTION

It is an object of the present invention to provide a method offabricating NROM cells and NROM cell arrays with improved dataretention.

There is therefore provided, in accordance with a preferred embodimentof the present invention, a method of fabricating an oxide-nitride-oxide(ONO) layer in a memory cell to retain charge in the nitride layer. Themethod includes the steps of forming a bottom oxide layer on asubstrate, depositing a nitride layer and oxidizing a top oxide layer,thereby causing oxygen to be introduced into the nitride layer.

Alternatively, in accordance with a preferred embodiment of the presentinvention, the method includes the steps of forming a bottom oxide layeron a substrate, depositing a nitride layer, oxidizing a portion of a topoxide layer thereby causing oxygen to be introduced into the nitridelayer and depositing a remaining portion of the top oxide layer, therebyassisting in controlling the amount of oxygen introduced into thenitride layer.

Further, in accordance with a preferred embodiment of the presentinvention, the method includes the steps of forming a bottom oxide layeron a substrate, depositing a nitride layer, depositing a portion of atop oxide layer and oxidizing a remaining portion of the top oxidelayer, thereby causing oxygen to be introduced into the nitride layer.

There is provided, in accordance with a preferred embodiment of thepresent invention, a method for improving the charge retention in anitride layer of a memory chip. The method includes the steps ofdepositing a nitride layer and introducing oxygen into the nitridelayer.

Alternatively, in accordance with a preferred embodiment of the presentinvention, the method includes the steps of depositing a nitride layer,controlling the thickness of the deposited nitride layer and introducingoxygen into the nitride layer.

Further, in accordance with a preferred embodiment of the presentinvention, the method includes the steps of forming a bottom oxide layeron a substrate, depositing a nitride layer at a thickness approximate tothe final thickness after fabrication, depositing a portion of a topoxide layer and oxidizing a remaining portion of the top oxide layer,thereby assisting in controlling the introduction of oxygen into thenitride layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with thedrawings in which:

FIG. 1 is a schematic illustration of a prior art NROM memory cell;

FIG. 2 is a schematic illustration of the NROM memory chip after anoxide-nitride-oxide layer has been laid down;

FIG. 3A is a schematic illustration in top view of a bit line implantmask;

FIG. 3B a cross section of a portion of the memory array of the chip ofFIG. 2 after the mask of FIG. 3A is laid down and after etching away theexposed portions of the ONO layer leaving part of the bottom oxidelayer;

FIG. 3C shows the cross section of FIG. 3B after an implant of animpurity to form the bit lines in the memory array portion of the chipof FIG. 3B;

FIG. 4 shows in cross section the memory array portion of the chip ofFIG. 3C after oxidation of the bit lines;

FIG. 5 is a schematic illustration of an ONO protect mask for the memoryarray and periphery sections of the chip; and

FIGS. 6A and 6B are schematic illustrations of the memory array portionof the chip of the present invention after a polysilicon or polysilicidelayer 60 has been laid down, in top and side views, respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to FIGS. 2, 3A, 3B, 3C, 4, 5, 6A and 6B, whichillustrate the NROM fabrication method of the present invention. Similarreference numerals herein refer to similar elements. It is noted thatthe present invention covers the fabrication of the entire chip, whichincludes the NROM memory array portion and the complementary metal oxidesemiconductor (CMOS) periphery devices.

In the following discussion, the process of etching a layer, whichincludes placing photoresist on the layer, placing a mask on thephotoresist, etching wherever the mask is not and removing thephotoresist, will not be detailed.

The method begins with a standard complementary metal oxidesemiconductor (CMOS) initial process for preparing the substrate 10including N well formation and field oxide formation. A screen oxidelayer is then grown (not shown) on substrate 10 after which it isremoved, typically with a wet etch thereby to remove any residualnitride at the edge of the field. A typical thickness of the screenoxide layer is 200-400 Å

Substrate 10 is then overlaid with an ONO layer. A bottom oxide layer 30is grown over substrate 10 typically to a thickness of between 50 Å and150 Å in a low temperature oxidation operation. A typical oxidationtemperature is about 800° C. but it can vary between 750-1000° C. Apreferred thickness of the bottom oxide layer 30 is 80 Å.

A nitride layer 32 is then deposited over bottom oxide layer 30 to athickness of between 20 Å and 150 Å where a preferred thickness is asthin as possible, such as 10 Å-50 Å. Applicant notes that a thin layerof the nitride prohibits lateral movement of the charge retained withinthe nitride, and hence, it is beneficial to control the thickness ofnitride layer 32.

Top oxide 34 is then produced either through oxidation of the nitride(i.e. growing of the oxide), or by deposition or by a combinationthereof. It is noted that top oxide 34 consumes nitride duringoxidation, where typically half of the oxide thickness comes from theconsumed nitride. Thus, if it is desired to have a top oxide which is100 Å thick, the nitride layer 32 should be at least 50 Å thicker thanthe final desired nitride thickness, with this extra nitride being forconsumption in the formation of the top oxide layer.

It is also noted that, during oxidation of top oxide layer, some of theoxygen is introduced into the non-consumed nitride layer.

Ultimately, as is described hereinbelow, nitride layer 32 is transformedinto nitride section 17, which provides the charge retention mechanismfor the memory cell. Nitride, particularly due to its structure, trapsthe electrons, which are introduced into nitride section 17. Oxygenhowever, is a better insulator than nitride and helps to minimize thelateral movement of electrons in nitride layer 32. It is thus animportant element for effective retention of the charge. It is thereforenoted that one of the factors effecting the quality of retention abilityof nitride section 17 is the concentration of oxygen within nitridelayer 32. The oxygen concentration is defined as the percentage ofoxygen atoms relative to the nitride atoms, irrespective of the type ofmolecule in which the oxygen atoms are found. The concentration canrange from a low of 10% to a high of 80%.

Hence, in order to produce a retention layer, which provides effectivecharge retention, it is recommended to introduce a high percentage ofoxygen into the nitride. Nonetheless, if the oxi-nitride composition istoo oxygen rich, even though nitride is essentially an oxidationbarrier, a run-away situation is produced whereby nitride layer 32absorbs too much oxygen and ceases to act as a barrier for oxygendiffusion. In such an instance, the oxygen introduced into the oxygenrich nitride layer 32 reaches the silicone oxide layer 18, and becomeSIO2.

In summary, in order to produce a nitride section 17 with maximumretention qualities, it is desirable to make nitride layer 32 as thin aspossible, with the maximum oxygen concentration, without inducing arun-away situation. Consequently, it is critical to control thefabrication the ONO structure, and specifically, the manner in which thetop oxide 34 is produced.

The top oxide is typically of a thickness of between 50 Å and 150 Å.Three alternative operations for creating a top oxide 34 of 100 Å aredescribed hereinbelow.

The first method involves depositing nitride layer 32 of approximately150-160 Å, growing 120-130 Å of top oxide 34, (which includes consuming60-65 Å of nitride layer 32) and removing 20-30 Å of oxide layer 34during cleaning. Since a large portion of nitride layer 32 is consumed,it is difficult to control the amount of oxygen introduced into nitridelayer 32. Thus, in order to avoid a possibility of run-away conditionsin the nitride layer, it is essential to “leave” a thicker nitridelayer. This alternative produces a thicker nitride layer; however itprovides for high introduction of oxygen into the nitride and is asimple process to perform.

The second method involves depositing nitride layer 32 at a thickness ofapproximately 60 Å, growing a thin layer of oxide layer 34(approximately 40 Å) while consuming about 20 Å of nitride, depositing80-90 Å and removing 20-30 Å during cleaning. Since depositing oxide isa quicker process than growing oxide, this alternative is quicker thanthe first alternative and it offers marginally better control over theamount of oxygen introduced into nitride layer 32

It is noted that the longer the oxidation process continues the greaterthe effect on previously produced layers. Therefore, in order diminishthe effect on previous layers, it is desirable to create the top oxidelayer as quickly as possible.

The third method involves depositing nitride layer 32 at a thicknessclose to the preferred final thickness, such as 20 Å, depositing 100-110Å of oxide, growing 2-5 Å of oxide and removing 20-30 Å of oxide duringcleaning. When growing oxide after it has been deposited, the depositedlayer acts as a barrier between the growing oxide and nitride layer 32.Hence, the oxygen is introduced slowly into nitride layer 32. Thisalternative is slower than the previous alternatives; however, itprovides a thin nitride layer and a more controlled manner forregulating the introduction of oxygen into the nitride layer.

The process by which the nitride and top oxide layers are generateddepends on the ability of the manufacturing facility to control thethickness and composition of the layers of the ONO structure.

At this point, the entire substrate 10 is covered with an ONO layer, asshown in FIG. 2. The next step involves depositing a bit line mask 40(typically photoresist 42 patterned in a well known manner), whoselayout within the memory array portion of the chip is shown in FIG. 3A,to create the bit lines, forming lines of sources and lines of drains.FIG. 3B illustrates a portion of the resultant chip within the memoryarray portion with the photoresist 42 patterned. FIG. 3B is a side view(similar to FIG. 2) with the columns 42 of the bit line mask in place.Photoresist columns 42 define the areas where the bit lines are not tobe implanted (i.e. the locations of the channels 15 (FIG. 1)).

Prior to implanting the bit lines, the top oxide and nitride layers 32and 34, respectively, are etched away from the areas between columns 42.The etch operation is typically a dry etch which might also etch aportion 44 of bottom oxide layer 30 which is between columns 42, leavingportion 44 with a predetermined thickness, such as 50 Å. The etchoperation produces oxide sections 18 and 20 and nitride section 17 undereach column 42.

After the etch operation, bit lines 12 are implanted (FIG. 3C) in theareas between columns 42. A typical implant might be 2-4×1015/cm2 ofArsenic at 50 Kev. It will be appreciated that this is a self-alignedimplant in which the bit lines are self-aligned to the ONO structures.

The photoresist layer 42 is then removed and bit line oxides 50 (FIG. 4)are then thermally grown over the bit lines 12 in an oxidationoperation. At the same time, side oxides 51, typically of 30 Å, aregrown along the sides of nitride layers 17 to improve data retentionwithin the nitride layers. The oxidation typically occurs in the rangeof 800° C. to 950° C. but preferably at the lower side of this range tominimize the diffusion of the bit line impurity while maximizing thethickness of the thermal oxide. This lowers the bit line capacitance.The oxidation temperature also activates the implanted bit lineimpurities.

Thus the typical oxidation process is a low temperature oxidation ofabout 800° C. which, on a P-substrate, normally is continued for a timesufficient to grow the equivalent of 100 Å of thermal oxide. On the chipof the present invention, however, top oxide sections 20 will notsignificantly increase in thickness during the bit line oxidation due tothe close presence of nitride sections 18 while oxide layer 44 over thebit lines 12 will increase significantly due to the presence of Arsenicin the bit lines 12. The result is that the bit line oxides 50 aretypically very thick, such as 500 Å thick, thereby lowering the bit linecapacitance.

It will be appreciated that the present invention separates the creationof bottom oxide sections 18 (and thus, of the entire ONO structure 16)from the creation of bit line oxides 50. Bottom oxide sections 18 arecreated over the entire array as part of creating the ONO structures.Bit line oxides 50 are created during the bit line oxidation operationand this oxidation does not significantly affect the oxide layers in theONO structures. Furthermore, bit line oxides 50 are self-aligned to theONO structures and, since the oxidation operation is at a relatively lowtemperature, bit lines 12 do not significantly diffuse into substrate 10during the oxidation operation.

It will further be appreciated that the ONO layers have been laid downon the entire chip and thus, are present in the periphery. In accordancewith a preferred embodiment of the present invention, the ONO layers canbe utilized as thick gate oxides in the portions of the periphery wherethicker oxides are needed. Thus, if two gate dielectric thicknesses arerequired in the periphery, the present invention provides one gatedielectric using the ONO layers and the second, thinner gate dielectriccan be produced in a separate gate oxide production step. Furthermore,as shown in FIG. 5, a single mask 52 can be utilized to mark both thelocations 54 of the thick gate oxides as well as to protect the memoryarray (area 56) while etching and oxidizing the periphery.

Mask 52 can be utilized in one of two alternative ways. In the firstembodiment, a threshold level adjustment implant for the peripheraltransistors is performed after mask 52 is laid down and patterned. Thisprovides the periphery with a threshold level different from that of thememory array area 56. In the second embodiment, the threshold leveladjustment implant is performed on the entire chip prior to laying downmask 52. In this embodiment, mask 52 serves only to mark the locationswhere the ONO layers are to be removed.

Specifically, in the first embodiment, after mask 52 is laid down, thethreshold voltage level adjustment is performed. This procedure involvesimplanting boron through the ONO layers into the portions of theperiphery of the chip not covered by mask 52. Typically, there are twoadjustment steps, one each for the n-channel and p-channel transistors.It will be appreciated that, in accordance with a preferred embodimentof the present invention, the adjustment implant is performed throughthe ONO layers since they are not yet capped and thus, do not block theimplant operation. It will further be appreciated that, for thethreshold adjustment procedure, the to-be-removed ONO layers act as asacrificial oxide (e.g. an oxide grown for an implant operation andimmediately thereafter removed).

Following the threshold voltage adjustment procedure, the ONO layers onthe unmasked portions of the chip are removed. Initially, a dry oxideetch is utilized to remove top oxide 34 and nitride 32 layers afterwhich a wet etch is utilized to remove bottom oxide layer 30. Followingremoval of mask 52, a gate oxide (not shown) of typically 100-150 Å isthermally grown over the entire chip. Due to the presence of nitride inthe memory array, the gate oxide step does not significantly affect thethickness of top oxide 20. However, this step creates gate oxides forthe transistors in the periphery.

It will be appreciated that the gate oxide thickness is thus independentof the thicknesses of the bit line oxide 50 and top oxide 20.

In a second embodiment, mask 52 is laid down after the gate andthreshold voltage level adjustment procedure is performed. Thus, thememory array portion of the chip also receives threshold leveladjustments. With mask 52 in place, the ONO layers on the unmaskedportions of the chip are removed, as described hereinabove. Once again,the ONO layers act as a sacrificial oxide, eliminating the necessity forthe additional sacrificial oxide operations.

Finally, following removal of mask 52, a gate oxide is grown over theentire array, creating gate oxides in the periphery only.

Following the gate oxide growth step, a polysilicon layer, which willcreate word lines for the memory array portion and will create gates forthe periphery transistors, is laid down over the chip. If desired, a lowresistive silicide, as is known in the art, can be deposited over thepolysilicon layer in order to reduce its resistivity. This creates a“polysilicide” layer. A typical total thickness of the polysilicidemight be 0.3-0.4 μm. As indicated by FIG. 6A, the polysilicide orpolysilicon layer is then etched using a mask into word lines 60 withinthe memory array. Typically the word line etch also etches at least thetop oxide 20 and the nitride 17 from between the word lines 60. Thisimproves the charge retention of the memory cells by isolating thenitride layers 17 of each transistor.

FIG. 6B illustrates one row of the resultant memory array in side view.The polysilicide or polysilicon layer 60 lies on top of the ONOstructures 16 (FIG. 4), thereby forming the gates 22 (FIG. 1) of theNROM cells. Bit line oxides 50 are thick enough to isolate neighboringONO structures 16.

The memory chip is then finished in the standard ways, including a sidewall oxidation step (typically a self-aligned step), a lightly dopeddrain (Ldd) implant procedure into the CMOS periphery only and a spacerdeposition. FIG. 6A illustrates the location of the spacers 62 as beingalong the sidewalls of the polysilicon word lines 60. The Ldd typicallyrequires separate masks for the n-channel and p-channel peripherytransistors.

It will be appreciated that, in the present invention, the thicknessesof the various elements of the NROM cell are generally independent ofeach other. For example, the thicknesses of the bottom oxide, nitrideand top oxide layers are typically selected as a function of the desiredoperation of the memory array, the bit line oxide is independent of thethickness of bottom oxide ONO structure and the gate oxide of theperiphery is independent of the other two oxide (i.e., the bit lineoxide and the bottom ONO oxide) thicknesses.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather the scope of the present invention isdefined only by the claims which follow:

1. A non-volatile memory cell comprising: a silicon based dielectriccharge trapping layer having a thickness of between approximately 10 and50 angstroms, wherein said dielectric charge trapping layer includes asubstantially homogenous distribution of a non-native and non-siliconebased impurity, at least a portion of said layer located substantiallybelow a gate of the cell; and an oxide layer below at least a portion ofsaid charge trapping layer.